Optical communication requires a mechanism for switching optical signals on and off in accordance with binary signals (i.e., 0 or 1). One of the simplest mechanisms is to directly control on and off of output of a semiconductor laser used as a light source. However, directly controlling on and off of the output of the semiconductor laser deteriorates oscillation stability of the semiconductor laser and above all high-speed swiching as high as 10 Gbps (giga-bits/second) or more is difficult to achieve.
Another technique for controlling on and off of an optical signal is to use an optical switch. According to this technique, constant optical outputs from a semiconductor laser are modulated to be on and off by a device provided at a subsequent stage. This device is referred to as an optical switch. By using an optical switch, light intensity of the semiconductor laser light source can be kept constant. Therefore, a problem of unstable oscillation can be solved. The performance of the optical communication is not influenced by the modulation rate of the semiconductor laser itself.
Conventional optical switches include a compound semiconductor. Examples of known operation principles include:
one that utilizes quantum confined Stark effect that leads to a red shift of effective band gap by applying an electric field to a multi quantum well structure;
one that utilizes change in a refractive index of an active layer due to current injection or a reverse bias voltage application; and
one that utilizes optical Kerr effect which is a third-order nonlinear optical effect.
The principle that utilizes quantum confined Stark effect allows some degree of design freedom according to a combination of well layers and barrier layers. However, the number of materials that can be selected by their intrinsic properties such as band-gap energy is limited, which results in limited operating wavelength range.
According to the principle that utilizes change in the refractive index, obtainable change in the refractive index is small. Therefore, in order to obtain sufficiently large difference between the signal light intensities of ON and OFF states, the length of the optical path needs to be as long as about several-hundreds of microns, which interferes with high-density integration.
An optical switch that utilizes optical Kerr effect is most likely to achieve high speed. However, this switch requires an additional high-power control light source for obtaining the optical Kerr effect, thereby rendering itself impractical.
As an optical switch that may overcome the above-mentioned problems, an optical switch that uses photonic crystal has recently been proposed. As one of the publications relating to the optical switch incorporating the photonic crystal, for example, Japanese Patent Laid-Open Application No. 10-90638 discloses a structure of an optical switch. This optical switch includes: a photonic band structure that forms a alternately structure on a two-dimensional plane with two types of optical media having different complex refractive indices where at least one of the optical media is a semiconductor; controlled light parallel to the two-dimensional plane; and means for radiating circularly-polarized light as control light to the alternately structure on the two-dimensional plane along an optical path that does not cross with an optical path of the controlled light, wherein the optical switch changes light transmission of the controlled light according to the control light.
A photonic crystal has at least two media with different refractive indices alternately and regularly arranged at a three- or two-dimensional on the order of the wavelength of light, namely, at a cycle of sub-micron meter. A photonic crystal has high design freedom and thus is receiving great attention as an artificial optical crystal having specific optical property.
FIG. 1A is a schematic view showing a two-dimensional photonic crystal as an exemplary structure of the photonic crystal. Here, cylinders made of a second medium 12 are imbedded in a first medium 11 in a two-dimensional triangular lattice arrangement. Such photonic crystal is known to form an energy band structure in response to the light wave present in the photonic crystal.
FIG. 1B is a schematic view showing a first Brillouin zone in a reciprocal lattice space corresponding to the triangular lattice shown in FIG. 1A. Point J is a vertex of the regular hexagon, point X is the midpoint of each side of the hexagon, and is the center of the hexagon.
The solid lines in FIG. 2 represent the results obtained by calculating an energy band structure of the photonic crystal shown in FIG. 1A in −X direction with respect to TM-polarized wave. The vertical axis of the diagram represents normalized Ω=ωa/2πc, where a is a lattice pitch (or a lattice constant), c is light velocity in a vacuum state, ω is angular frequency of the light wave, and k represents the magnitude of the wave vector.
In the energy band structure shown in FIG. 2, light wave mode for light traveling in the −X direction does not exist in an energy range of Ω1-Ω2, where it is referred to as a “photonic band gap (PBG).”
The above-described optical switch disclosed in the Japanese Patent Laid-Open Application No. 10-90638 controls on and off of light propagating in the photonic crystal by controlling the PBG according to the change in the refractive index of the material. In this conventional example, since light with energy Ω3 is within the PGB, light cannot be propagated in the OFF state. The refractive index of the photonic crystal material is changed by externally radiating control light or by injecting current to the photonic crystal. As a result, the energy band structure defined by the solid lines in FIG. 2 alters to an energy band structure defined by the dashed lines where a propagation mode becomes effective for the light of Ω3 in the ON state. On the contrary, change in the refractive index alters the state of the light wave with energy of Ω4 from ON state to OFF state.
The above-described optical switch using a photonic crystal disclosed in Japanese Patent Laid-Open Application No. 10-90638 utilizes a small change in the refractive indices of the media. By setting the wavelength of the controlled light to the edge of the energy band, ON/OFF effect of the light (i.e., function of the optical switch) can be obtained.
The above-described optical switch using a conventional photonic crystal simply utilizes characteristic of the photonic crystal itself as a bulk structure, but have no regard to the guided wave of light necessary for actually forming an optical circuit, that is, optical confinement in a direction perpendicular to the light propagating direction. Therefore, a recovery efficiency of light that has propagated through the photonic crystal is considered to be extremely low.
In order to guide the wave of light efficiently, the photonic crystal may be shaped into a narrow waveguide with a width of 1 to 2 μm with which a basic propagation mode can be achieved. In this case, however, the energy band of the bulk structure no longer retains its shape shown in FIG. 2, and thus the photonic band gap necessary for ON/OFF control of the propagated light is no longer formed.
All of the above-described conventional optical switches control ON and OFF of a single channel with a single switch. Therefore, when a multi-channel configuration is considered, a plurality of optical switches need to be prepared for the number of channels.
Furthermore, the conventional optical switches control entire ON/OFF of light according to control signals such as current, voltage and light. Therefore, when the waveform of the control signal is deteriorated, the waveform of the light is deteriorated as well.
Thus, the present invention has an objective of providing an optical switch having a photonic crystal structure, which can propagate light efficiently and which can operate at high speed.
As can be appreciated from the following description, the present invention also has an objective of providing an optical switch, where a multi-channel optical switch that can switch among a plurality of channels with a single switch is realized to make a compact multi-channel switch, or where a waveform shaping function is realized that makes the light intensity to be predetermined range, when the signals are ON.